Previous attempts at controlling such vibrations are disclosed in U.S. Pat. No. 4,635,892, "Active Vibration Suppressor" to Baker; U.S. Pat. No. 4,819,182, "Method and Apparatus for Reducing Vibration of a Helicopter Fuselage" to King et al. and U.S. Pat. No. 4,950,966, "Adaptive Vibration Canceller" to Moulds. In addition, another system is shown and described in U.K. Patent Application GB 2165667A, "Method of reducing the transmission of vibrations" to Redman-White et al.
Methods for controlling vibrations in flexible structures have been recently deemed to have commercial merit such as active engine mounts. The techniques utilized have merit when isolating vibrations of one structure from another structure. The key to doing this is the co-location of control force and sensing means which simplify the problem and make the instant system much better than prior systems.
The active mount takes advantage of the co-location of the vibration "footprint" and the control force and the sensing means at the connection point. However, many technical difficulties exist in the implementation of an active mount. One problem in particular is the coupling of the actuator and resilient means. This problem becomes more troublesome when multiple degrees of freedom are attempted. Thus, it is desirable to have a system in which the benefits of co-location of actuation and sensing is accomplished while avoiding the complexities of active mount integration. The latter goal is particularly important in retrofit situations. For example, in order to actively control the vibration from the engine of a pleasure craft as it enters the hull, active mounts could be used. However, the engine would need re-alignment upon reinstallation. The use of the subject invention would avoid this situation.
It is critical that all connections to a structure (or at least the degrees of freedom in which significant vibration is transmitted) be controlled with the subject invention. In this way all force inputs to the base structure will be controlled. Co-location of the sensor and force producing means will serve to match the "disturbance" input with the control input thereby effecting the modes of the base structure in the same manner, thereby controlling the structure's response.
This invention is intended for use in applications of mounting machinery to flexible structures at discrete mounting points. The discrete mounting point may be either a passive mount, or a direct connection. A passive mount will improve the high-frequency, broadband performance of the system. The actuator in this invention should be any device which uses the inertia of a moving mass to create a force. It should be noted that the sensor and actuator can be integrated into a common package which is then attached to the structure.
Examples of controllers which can be used are shown and described in U.S. Pat. Nos. 4,862,506 to Landgarten et al, 4,878,188 to Ziegler, 5,105,377 to Ziegler, and 5,091,953 to Tretter, all of which are hereby incorporated herein by reference. Other types of adaptive and non-adaptive control systems may be used.
The method of sensing should be a measure of force, preferably a force gauge (such as a PVDF film), or an accelerometer. Displacement or velocity sensing is also possible, however force is the preferred measure. The placement of the sensor in the same "footprint" as the actuator and disturbance source is critical as shown in FIG. 2.
The advantages of co-location sensing and actuation are of critical importance in active vibration control. King describes a system in which a plurality of sensors and actuators are placed around a vehicle in order to alter the structural response of the vehicle. Because the sensors and actuators of King's system are not co-located the disturbance input is not minimized, but the response at the sensor locations are. This indicates that the structural response of the vehicle may be adversely affected at locations other than those of the sensors. I.e., the need for co-location results from the need to effect or force the sub-structure in the same manner as the disturbing input.
The subject invention would place an inertial actuator near the connection points of the disturbance (for example, the engines), and would sense the response of the structure at or near the same point. The concept of "or near" the same point is crucial to this invention. The system (sensor and actuator) must be able to couple into the structural modes of the vehicle in very nearly the same way as the disturbance source. A multiple input, multiple output controller would be used to minimize the force transmission into the vehicle at the point of the disturbance input.
In some, the Redman-White et al application, a system is used which controls both translational and rotational degrees of freedom in a structure. This 2 DOF type of system is critical when controlling flexural wave motion in a structure, however, it is not a configuration where the sensors and actuators are co-located, as they are only anticipating the need to control flexural waves in a well defined structural path. It does not anticipate the subject invention as there is no need to couple into the structure in the same fashion as the disturbance source. It merely attempts to create a certain boundary condition at the point where the sensors are located.
The Baker patent discusses a system in which an inertial actuator is used to reduce the response of a structure at the point of sensing. Zero motion (in one degree of freedom) does not imply the overall reduction of the structural response. As in King, the response of the structure may be worsened in other locations. It is also clear from the disclosure that the need for spatial matching of the disturbance is essential.
The Moulds patent discloses an active vibration canceller, primarily for use in rotating machinery with magnetic bearings. Moulds claims a reaction mass actuator for imparting a control force. However, in several claims it states that the actuator should be at a location other than that of the sensor. Again it does not anticipate the importance of spatial matching the vibrational input of the disturbance source with that of the cancellation force.
Accordingly, it is an object of this invention to provide an improved co-location sensing and actuation system for use in isolating vibrations of a structure.